In the field of oceanography, it is often desired to obtain a direct measurement of the identity and pressure of dissolved gases present in seawater or freshwater. A major target gas is carbon dioxide —CO2. Known techniques for obtaining such measurements include the use of semi-permeable membranes that allow such gases to penetrate into a sampling volume, while resisting the penetration of the external liquid and sustaining the hydrostatic pressure so that it does not collapse the sampling volume.
Samples of prior art techniques for measuring the partial pressure of gases, and particularly carbon dioxide dissolved in seawater, are respectively European patents EP-01 04 35 85 A2 and EP-00 59 26 32 A1. An example of a planar membrane which is supported in order to resist hydrostatic pressure is described in U.S. Pat. No. 5,121,627. The support of the planar geometry of a membrane against substantial hydrostatic pressures, while preserving structural integrity and semi-permeability of the membrane, is difficult, and is impracticable in applications where a relatively large total area of membrane is required for an acceptably high rate of penetration of the gas under detection into the sampling volume, with corresponding enhancement of response time performance.
Use of semi-permeable membranes in the format of tubing is described in U.S. Pat. Nos. 3,871,228; 4,563,892, and 4,662,210. An advantage of the use of tubing is that, particularly in respect of smaller diameter tubing, the curved walls of semi-permeable membrane material can be largely self-supporting, at least at lower pressures. Thus, the area limitations of the planar membrane can be overcome, in that the surface of the tubular membrane forms the effective area for penetration of gas into the interior volume, and may be increased proportionally by the length of the tubing comprising the membrane.
However, at higher hydrostatic pressures, the semi-permeable material forming the tubular membrane does not have sufficient mechanical strength to resist the external pressure and will undergo collapse to close the interior space such that the penetration of the gas and its transfer to the detection instrumentation is obstructed. It is also known that to create a optimal system for the detection and measurement of gas by penetration of the gas from a liquid through a semi-permeable membrane into a sample space, the volume of the sample space must be relatively small in relation to the active membrane area to achieve reasonably rapid equilibration of the gas in the sample space with the ambient liquid from which it was derived, thereby shortening the response time of the instrumentation.
In U.S. Pat. No. 5,763,762, it is suggested that a filler for the purpose of reducing the internal volume of a tubular semi-permeable membrane may be provided from the group: thread, mono-filament, powder, wire, in situ formed polymer, fluid and any combination of these. However, U.S. Pat. No. 5,763,762, does not discuss as a design consideration, the utility of a system for the detection or measurement of a gas dissolved in liquid under pressure, or the incorporation of the feature of a pressure resistance membrane as an element of the invention claims.
Fillers of the type previously proposed will have a tendency to permit partial or complete collapse of the tubing, the compression and constriction of the filler substrate, or the rendering the membrane susceptible to mechanical penetration or rupture by the forcing of the membrane film into voids in the filler substrate or projections on filler elements, thereby increasing the pressure drop occurring when sampled gas is removed from the core of such tubing or rendering the system non-functional.
Further, in order to shorten the response time of such a system, it is desirable for the gases present in the external liquid to be exposed to the semi-permeable membrane with minimal interference from boundary layers that are depleted in such gases.
The invention addresses the objects of overcoming the disadvantages of the prior art and provides for a new form of gas detection and transfer system that operates with a minimized response time.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.